Torsional vibration dampers are employed extensively in internal combustion engines to reduce torsional vibrations delivered to rotatable shafts. The torsional vibrations may have a considerable amplitude and, if not abated, can potentially damage gears or similar structures attached to the rotatable shaft and cause fatigue failure of the rotatable shaft. Torsional vibration dampers reduce the amplitude of the vibrations by converting the vibrational energy to thermal energy as a result of the damping action. The absorption of the vibrational energy lowers the strength requirements of the rotatable shaft and, therefore, lowers the required weight of the shaft. The torsional vibration damper also has a direct effect on inhibiting vibration of nearby components of the internal combustion engine which would be affected by the vibration.
Virtually all motor vehicles with internal combustion engines incorporate a “serpentine” drive belt system consisting of a single endless drive belt and a series of pulleys. The pulleys derive power from the endless drive belt and operate to drive the various vehicle accessories such as the engine fan, power steering pump, air pumps, air conditioning unit, and the alternator. The endless drive belt that drives each of these pulleys is driven by a drive pulley connected to the crankshaft of the internal combustion engine. To reduce the transfer of vibrations between the crankshaft and the serpentine drive belt system, the drive pulley may comprise a torsional vibration damper that functions to reduces the amplitude or magnitude of the angular vibrations delivered by the crankshaft.
Torsional vibration dampers have been fabricated with a composite central hub consisting of an outer polymeric annular body surrounding an inner metallic insert. Conventional torsional vibration dampers having such composite central hubs are susceptible to irreversible structural damage when removed from the crankshaft to service the internal combustion engine. Typically, a gear puller is utilized which applies a lateral force sufficient to remove or pull the torsional vibration damper from the crankshaft. The applied lateral force is significant in those circumstances in which the torsional vibration damper is frictionally fit to the crankshaft or in which corrosion, deformation, or the like has increased the character of the engagement therebetween. One failure mode is catastrophic mechanical damage to the polymeric material in portions of the annular body grasped by the gear puller. Such mechanical damage can permanently unbalance the annular body and degrade the performance of the torsional vibration damper. Another failure mode is separation of the polymeric annular body from the insert in response to the lateral force applied by the gear puller. In that instance, the annular body is detached from the insert, which remains attached to the rotatable shaft, and the torsional vibration damper is irreversibly damaged.
Another deficiency of conventional torsional vibration dampers is that the polymeric annular body may loosen from the metallic insert and result in breakaway or slip of the annular body relative to the insert. If the torsional vibration damper slips, the transfer of power from the crankshaft to the damper will be reduced in proportion to the slippage and the operation of the vehicle accessories will be impaired. In addition, the torsional vibration damper will no longer operate in an optimum fashion for damping vibrations. In instances of breakaway, the vehicle accessories powered by the belt system will no longer have a drive connection to the engine and become inoperable. As the polymeric material of the damper ages or if the damper is exposed to excess loading, the polymeric annular body is more likely to loosen or decouple from the insert.
The probability that a conventional composite torsional vibration damper will fail during operation, such as by decoupling of the polymeric body from the insert, is increased at elevated operating temperatures. All polymeric solids, including solids formed of crystalline polymers, contain an amorphous portion that experiences a glass transition. At the so-called glass transition temperature, the amorphous polymer changes from a hard, brittle form to a material which is soft and flexible. At higher temperatures, the crystalline portion of the polymer will melt. To maintain a drive connection between the crankshaft and the serpentine drive belt system, the polymer forming the annular body of the torsional vibration damper must remain a hard and brittle solid at the temperature of the operating environment. Polymers utilized in conventional torsional vibration dampers tend to soften and fail in the automotive temperature environment of the crankshaft, which can rise as high as 230° F.
There is a need for a composite torsional vibration damper for a rotatable shaft that can be removed from the shaft without inflicting significant mechanical damage to the damper, that can operate dependably at elevated temperatures, and that will exhibit a reliable mechanical interconnection to prevent rotation of the outer polymeric annular body relative to the inner insert.